Many portable, battery-powered devices (e.g. mobile handsets, keyboards, personal digital assistants) recharge their batteries through dedicated charging ports such as an AC-to-DC (alternating current to direct current) wall-wart adapter. A wall-wart adapter is small power supply brick with an integral male plug. The wall-wart is designed to plug directly into a wall outlet. It is called a “wart” because when it is installed on a power strip it tends to block at least one more socket than it uses. These portable, battery-powered devices may also recharge their batteries through a standard host port. A standard host port usually has a five volt power supply provided from the USB (universal serial bus) ports on PCs (personal computers) such as laptop, desktop and notebook PCs. Commonly, the portable device has only one input which (s designed for USB compatibility in order to save space (as opposed to having a separate wall-wart connector).
Usually, a standard host port has a current limit of 500 ma (milliamps) while a BCS (Battery Charging Specification) compliant host port can provide much more current. For example, some BCS compliant host ports can provide 1.5 amps to a portable device. Because many portable devices cannot differentiate when they are plugged into a standard host port versus a BCS compliant host port, most portable devices limit the amount of current they draw to ensure that the portable devices do not overload the host port.
Several recent standards have defined protocols that allow a portable device to differentiate between different power sources to allow the portable device to drawn maximum current from a host port. Drawing maximum power from a host port reduces the time required to charge the portable device. Several recently released industry standards such as the Chinese PRC Telecommunications Industry Standard YD/T 1591-2006 and USB 2.0 Battery Charging Specification 1.1 (BSC1.1) define both power sources, handshaking protocols and allowable current draws for host ports. Other proprietary-based schemes (such as Apple's method for charging iPods and iPhones) are company specific. These standards define how a portable device communicates with a host port to determine the maximum allowable current draw.